Disk drive memory systems have been used in computers for many years for the storage of digital information. Information is recorded on concentric tracks of a magnetic disk medium, the actual information being stored in the form of magnetic transitions within the medium. The disks themselves are rotatably mounted on a spindle. Information is accessed by a read/write transducer located on a pivoting arm that moves radially over the surface of the rotating disk. The read/write head or transducer must be accurately aligned with the storage tracks on the disk to ensure proper reading and writing of information.
During operation, the disks are rotated at very high speeds within an enclosed housing using an electric motor generally located inside a hub or below the disks, for example an in hub or in spindle motor. The spindle includes bearing components to support the rotation and axial location of the disc stack. Such motors may have a spindle mounted by two ball bearing systems to a motor shaft disposed in the center of the hub. However, with the decreasing size of information storage systems, other types of bearings including fluid dynamic bearings are being developed, such as those useful designs discussed herein.
In these types of bearings, a lubricating fluid, i.e., gas, liquid or air is used in the active bearing region to generate fluid dynamic pressure to prevent metal to metal contact.
The bearing region comprises two relatively rotating surfaces, at least one of which supports or has defined thereon pattern of grooves. The grooves collect fluid in the active bearing region. When the two surfaces of the bearing rotate relative to one another, a pressure profile is created in the gap due to hydrodynamic action. This profile establishes a stabilizing force so that the bearing surfaces rotate freely without contact. In a disc drive, the rotating surface is associated with a hub supporting one or more discs whose rotation and axial location is kept stable by the pressure profile.
The tangential forces created in the bearing area characterize the bearing with respect to changes in shear in the fluid and are summed up in torque, which in turn defines power consumption. The pressure profile defines all forces normal to the bearing surface which characterize the bearing with respect to axial load and radial and angular restoring forces and movement.
A specific fluid dynamic bearing design can be characterized by multiple qualities, including power consumption, damping, stiffness, stiffness ratios and restoring forces and moments.
The design of the fluid dynamic bearing and specifically the groove pattern, is adapted to enhance the stiffness and damping of the rotating system, which includes one or more discs rotating at very high speed. Stiffness is the changing force element per changing distance or gap; damping is the change force element per changing rate of distance or gap. Optimizing these measures reduces non-repeatable run out (NRRO), an important measure of disc drive performance.
A further important issue is the need to maintain the stiffness of the hydrodynamic bearing. The stiffer the bearing, the higher the natural frequencies in the radial and axial direction, so that the more stable is the track of the disc being rotated by a spindle on which reading and writing must occur. Thus the stiffness of the bearing in the absence of any mechanical contact between its relatively rotating parts becomes critical in the design of the bearing so that the rotating load can be stably and accurately supported on the spindle without wobble or tilt.
Current fluid dynamic bearing designs typically employ a combination of radial and axial bearings. The radial bearings typically are formed in a journal (i.e., between two relatively rotating components such as a shaft and a sleeve). Net hydraulic pressure created by the radial bearings establishes a thrust force on the end of the shaft that displaces the shaft axially. A further axial force is generated by a thrust bearing, which typically includes a groove surface located proximate the end of the shaft that is subjected to the thrust force. The combination of radial and axial bearings therefore helps to stabilize the motor.
In several current designs, the axial bearing is provided by a grooved thrust plate; however, a motor may conserve power by employing a thrust plate-less design in which an end surface of the shaft or a counter plate has a grooved surface that faces the end of the shaft to provide the axial bearing surface. In conventional motor designs, the counter plate incorporates a grooved pattern in which multiple chevron or spiral patterns are formed on the bearing surface, such as the patterns illustrated in FIGS. 1 and 2, respectively.
While the bearing groove patterns illustrated in FIGS. 1 and 2 have proven to be effective in providing a stabilizing axial bearing surface, recent testing has shown that such conventional designs do not always provide sufficient pressure to raise or effectively support the shaft off the counterplate surface as the diameter of the shaft decreases. A design is needed which works with slow relative speeds which is directly related to shaft diameter, because the diameter decreases, the number of grooves which can be formed to define the thrust bearing diminishes with some manufacturing processes; and the stiffness of the bearing falls as the number of grooves is lessened.
Therefore, a need exists for a low power fluid dynamic bearing design for a thrust bearing which is especially effective for smaller diameter shafts.